This invention relates to two-piece drill bits, having a shank portion and a drill portion.
Two-piece drill bits are known in the prior art, and commonly have a hexagonal shank for mounting in a suitable holder or chuck. Several different structures and methods of manufacture are known. For example:
(1) Cutting a hex shaft to length and drilling an axial hole at a distal end. A flat is ground/machined onto the shank of the drill part, which is inserted into the drilled hole of the hex shaft. The shaft is then swaged at the position where the flat is, so that the displaced material is lodged against the flat of the drill shank to secure it inside the hole of the hex shaft. There are at least two perceived disadvantages with this method. To enable insertion, the hole has to be drilled over-size to accept the drill shank. After swaging, the shank is shifted to one side, there is thus a loss of capacity of the drill with respect to the hex shaft. Furthermore, after swaging again one side of the hex shaft is flattened, resulting in loss of the hex profile at that part of the hex shaft. This may make it difficult for the drill bit assembly to be inserted into ordinary hex-shank holder tools available in the market.
(2) Cutting a hex shaft to length and drilling an axial hole at a distal end. Introducing adhesive in the drilled hole and inserting a drill shank by pressing and letting the adhesive dry. The method may not provide a drill bit having the required strength for most applications, because during heavier use, the drill shank may spin around inside the drilled hole of the hex shaft.
(3) Drill shanks are arranged and held in a die having cavities that are of the same shape as regular hex shafts. The shank of the drill bit is then cast (usually of Zinc/Zinc alloy) around the drill shanks and left to cool. Because the cast material is softer than the drill shanks, the shank of the drill bit wears prematurely and cannot maintain its shape after repeated use. Also the centring of the drill shank in the hex shaft can become disturbed after wearing out the shank material. Furthermore, after casting and removal of the die, there is usually left some extra material at the die parting line (usually on both sides of the hex shaft). These imperfections may cause improper positioning of the drill bit in drill chucks or tool hex mounts, with the subsequent run-out and concentricity problems during use.
(4) Cutting a hex shaft to length and drilling an axial hole at a distal end. The drill shank is then pressed into the hole and the shank is cross-drilled (radially through the shank) and a pin inserted through the radial hole to secure the drill shank. In case a smaller diameter drill is used, the radial hole is tapped so a set-screw may be used to retain the drill shank in the hex shaft. This method is time-consuming and expensive because of all the steps involved. Also, the method yields drill bit assemblies which are non-concentric and difficult to insert into hex tool holders, especially when the ends of the pin or set-screw are not flush with the outer flat surface of the hex shaft.
It is an object of the invention to provide economical and effective alternative structures and methods of manufacturing two-piece drill bits.
In the invention, an axial hole is drilled into the distal end of a preferably hexagonal shank. The proximal end of the drill portion is inserted in the axial hole and is secured in the hole by one or more of a variety of mechanical means as described in greater detail below.
In the preferred embodiment, at least a portion of the proximal end section of the drill portion is knurled, and the shank is compressed tightly against the proximal end section of the drill portion, for example by rolling or roll-peening.
In the method of the invention, a knurled portion is formed on at least a portion of the proximal end section of the drill portion, shanks of a suitable length are drilled with an axial hole at a distal end, the proximal end section of the drill portion is inserted into the axial hole, and at least a portion of the shank is mechanically deformed to symmetrically compress the shank onto the drill portion, for example by rolling, roll-peening, or differential expansion in heating and cooling.
Additional features of the invention will be described or will become apparent in the course of the following detailed description.